1. Field of the Invention
This invention relates to photoelectric sensors capable of avoiding erroneous operation caused by mutual interference.
2. Description of the Related Art
Erroneous operation caused by mutual interference of a plurality of sequentially arranged photoelectric sensors and erroneous operation caused by illumination light having alternating-current (AC) harmonic components, such as light of inverter fluorescent lamps, have been issues in the related art. In addition, the use of light-emitting-diode (LED) illumination has recently been spreading. Since LED illumination often has AC harmonic components, the likelihood of photoelectric sensors operating erroneously is expected to increase.
To cope with the issues described above, there are photoelectric sensors that avoid erroneous operation caused by a disturbance having a pulse waveform and a disturbance having an AC waveform (see, for example, Japanese Examined Patent Application Publication No. 62-7733 and Japanese Unexamined Patent Application Publication Nos. 63-263917 and 2003-23347). A photoelectric sensor disclosed in Japanese Examined Patent Application Publication No. 62-7733 delays transmission of light by a predetermined period if a disturbance is detected. In addition, a photoelectric sensor disclosed in Japanese Unexamined Patent Application Publication No. 63-263917 transmits light while changing the light transmission cycle.
Further, a photoelectric sensor disclosed in Japanese Unexamined Patent Application Publication No. 2003-23347 detects a disturbance having an AC waveform by providing positive and negative thresholds in addition to a threshold used to determine the presence or absence of a target object. If a disturbance having an AC waveform is detected, the photoelectric sensor receives light at a zero-cross timing of the AC waveform. If no disturbance is detected for a predetermined period, the mode is switched into another mode. If a disturbance is detected in the other mode, transmission of light is delayed by a predetermined period.
The method disclosed in Japanese Examined Patent Application Publication No. 62-7733, however, is less effective against mutual interference. Specifically, since strong disturbance light is input when mutual interference of photoelectric sensors occurs, a received light signal does not have a pulse waveform but rather has a fluctuating waveform in which the signal fluctuates a plurality of times. For this reason, even if light is transmitted after a predetermined period as in the method disclosed in Japanese Examined Patent Application Publication No. 62-7733, a light signal supposed to be received may be lost due to saturation caused by the fluctuating waveform. In addition, the method disclosed in Japanese Examined Patent Application Publication No. 62-7733 is unable to avoid the influence of AC disturbance light.
The method disclosed in Japanese Unexamined Patent Application Publication No. 63-263917 is less effective against AC disturbance light. Specifically, light is transmitted while changing the light transmission cycle in the method disclosed in Japanese Unexamined Patent Application Publication No. 63-263917. Coincidence of reception timings of AC disturbance light and light of interest is inevitable, which may lead to erroneous operation.
The method disclosed in Japanese Unexamined Patent Application Publication No. 2003-23347 is less effective against mutual interference. Specifically, as described above, since strong disturbance light is input when mutual interference of photoelectric sensors occurs, a received light signal does not have a pulse waveform but rather has a fluctuating waveform. For this reason, even if light is transmitted after a predetermined period as in the method disclosed in Japanese Unexamined Patent Application Publication No. 2003-23347, the light signal supposed to be received may be lost due to superimposition of the fluctuating waveform.
In addition, the method disclosed in Japanese Unexamined Patent Application Publication No. 2003-23347 requires a comparator having a positive threshold and a comparator having a negative threshold in addition to a comparator having a threshold used to determine the presence or absence of a target object, increasing the scale of the circuit. Further, since the photoelectric sensor operates while switching between an AC disturbance light handling mode and a mutual interference handling mode, processing is complicated. The method disclosed in Japanese Unexamined Patent Application Publication No. 2003-23347 is to control the light transmission timing so that the light reception timing matches the zero-cross timing of the AC waveform of the disturbance. However, since the processing speed of timing control is limited, the noise frequency that can be coped with is limited. In addition, it is difficult to cause the light reception timing to completely match the zero-cross timing if the noise frequency is high, and erroneous operation may occur if the timing is too early and too late.